1. Field of the Invention
The present invention relates to a computer to which external display can be connected, and more particularly, to a computer which can determine whether or not an external display is connected to it, in accordance with the status data stored in a register, and also to a method of determining whether or not an external display is connected to a computer.
2. Description of the Related Art
In recent years, various portable personal computers, generally known as "lap-top personal computers", have been developed. The lap-top personal computers have a flat panel-type display such as a plasma display. The plasma display is hinged to the main body of the personal computer, and can be rotated between a closed position and an open position. When the plasma display is in the closed position, it is placed on the keyboard integral with the main body, rendering the entire computer portable. In view of this, the flat panel-type plasma display is desirable as a display for a lap-top personal computer which should be as compact as possible.
Some of the lap-top personal computers, which have been developed recently, are designed to be connected to an external cathode-ray tube (CRT) display, so that it can be used as a desk-top personal computer. Each of these lap-top personal computers has a connector for connecting the CRT display to the lap-top personal computer. Once the CRT display has been connected to this connector, data can be displayed on the CRT display, and can no longer be displayed on the plasma display. When the CRT display is not connected to the connector, data can be displayed on the plasma display.
Hence, which display, the plasma display or the CRT display, can display the data depends upon whether or not the CRT display is connected to the connector. The plasma display and the CRT display must be controlled in different methods to display the same data, by means of a display controller. To select an appropriate display-controlling method, it must be determine whether or not the CRT display is connected to the connector of the lap-top personal computer.
The connection or non-connection of the CRT display to the lap-top personal computer is determined by checking the fourth bit of the input status register incorporated in the display controller. The value of the fourth bit depends upon the total voltage of the specific pins of the connector. More precisely, when the CRT display is connected to the connector, the total voltage of these pins falls below a reference voltage, whereby status data "1" is set at the fourth bit of the input status register, thus indicating that the CRT display is connected to the lap-top personal computer. On the other hand, when the CRT display is disconnected to the connector, the total voltage of the specific pins becomes either equal to or higher than the reference voltage, whereby status data "0" is set at the fourth bit of the input status register, showing that the CRT display is not connected to the lap-top personal computer.
Therefore, when the fourth bit of the input status register has logic value of "1", it is determined that the CRT display is connected to the lap-top personal computer, and when this bit has logic value of "0", it is determined that the CRT display is not connected to the lap-top personal computer.
The fourth bit of the input status register represents the correct logic value while the personal computer remains in display mode, no matter whether or not the CRT display is connected to the personal computer. However, when the CRT display is not connected to the personal computer, the bit often represents a wrong logic value while the computer remains in non-display mode. Needless to say, while the computer is set in non-display mode, the display controller supplies no video signals to the plasma display or the CRT display. Without video signals supplied to the CRT display, the total voltage of the specific pins of the connector is lower than the reference voltage. In this case, the fourth bit of the input status register has logic value "1", representing that the CRT display is connected to the personal computer, which is not true. It should therefore be determined, while the computer is set in the display mode, whether or not the CRT display is connected to the lap-top personal computer, in accordance with the fourth bit of the input status register. Thus, in order to determine the connection or non-connection of the CRT display, it is necessary, first to detect that the personal computer is in a display mode, then to read the fourth bit of the input status register.
Here arises a problem. In the one horizontal scanning period (1H) for scanning one horizontal line on the display screen of the CRT display, the display mode period for displaying the data on the display screen is as short as about 25 .mu.s. It is next to impossible to detect, within such a short time, that the plasma display is in the display mode, and to read the fourth bit of the input status register. Even if it is detected that the CRT display is in the display mode within the one horizontal scanning period (1H), the fourth bit will be read, in most cases, after the display mode period has lapsed. Consequently, the fourth bit of the input status register, i.e., the status data, has logic value "1" even if the CRT is not connected to the lap-top personal computer. Thus it is inevitably determined that the CRT display is connected to the lap-top personal computer, which is not true. In brief, it has been difficult to determine, with accuracy, whether or not the CRT display is coupled to the personal computer.